Toner pickoff apparatus

ABSTRACT

Weakly charged toner particles are removed from a developer mix prior to developing an electrostatic latent image by bringing the developer in contact with a segmented biased roller. Also, toner that is charged to the wrong polarity may be removed from the developer mix by the same biased roller prior to developing the electrostatic latent image. The removal of the toner particles from the developer prior to presenting the developer to an electrostatic latent image substantially reduces background when developing the latent image.

O United States Patent 1 1111 3,865,080

Hudson Feb. 11, 1975 [54] TONER PICKOFF APPARATUS 3,634,077 1/1972 Sullivan 118/637 X 3,637,306 1/1972 Coo er 118/637 X 1 lnvemori Fredfrlck 3,649,262 3/1972 Cad et a1 118/637 x Henr e a, NY 3,662,711 5/1972 Hudson 118/637 [73] Assigneez xerox Corporation, Stamford 3,754,962 8/1973 Berller et a1 118/637 X Conn. Primary ExaminerMervm Stem [22] Wed: 1973 Assistant ExaminerLeo Millstein [21] Appl. No.: 324,466

[57] ABSTRACT {52] U.S. Cl. 118/637, 117/17.5 Weakly charged toner particles are removed from a [51] Int. Cl G03g 13/00 developer mix prior to developing an electrostatic la- 1 Field of Search /175; 118/636, 63 tent image by bringing the developer in contact with a 1l8/DIG. 24 segmented biased roller. Also, toner that is charged to the wrong polarity maybe removed from the devel- [56] References Cited oper mix by the same biased roller prior to developing UNITED STATES PATENTS the electrostatic latent image. The removal of the 3,411,482 11/1968 Brodie 118/D1G. 23 Particles from develqper Preseming 3,412,710 11/1968 RObinSOn.... 118/637 the developer to electrostatlc latent Image Substan- 3,536,042 10/1970 Weiler 118/637 tially reduces background when developing the latent 3,550,556 12/1970 Chawda.. 118/637 image. 3,574,301 4/1971 Bernhard 118/637 3,592,675 7/1971 Tung-nan Cheng 118/637 X 12 Clam, 5 Drawmg Figures 1 TONER PICKOFF APPARATUS DESCRIPTION OF THE INVENTION In developing electrostatic latent images, a developer material, comprising toner particles and carrier particles, is presented to the electrostatic latent image for developing the same. The toner particles are triboelectrically charged through a mixing and tumbling action with the carrier particles present in the developer mix. Normally, however there are some toner particles which carry a low charge and some toner particles which are charged to the wrong polarity. It has been found that background development level is a function of the concentration of improperly charged toner particles in the developer with background becoming worse with an increase in this type of toner. The reasons are not fully understood, but some general observations may be made. When positive-to-positive reproduction is desired, and the electrostatic latent image carries a positive charge, then the carrier will have a positive charge and the toner a negative charge. Normally, the electrostatic latent image may have a charge of +800 volts and the background will have a charge of around +200 volts and the toner will carry a negative charge.

One explanation of the above phenomenon is that assuming the electrostatic latent image carries a positive charge of +800 volts and a background charge of +200 volts, the toner particles of low negative charge will be attracted by both the background charge and the image charge while the more negatively charged toner will be preferentially strongly attracted by the high charge on the image. It is also felt that toner particles of low charge and charged to the wrong polarity are mechanically deposited on the background It is therefore an object of this invention to eliminate the above disadvantages and thereby suppress background development by withdrawing low or weakly charged toner particles from the developer prior to presenting the developer to an electrostatic latent image.

It is a further object of this invention to eliminate the above disadvantages and thereby suppress background by withdrawing toner particles, which are charged to the wrong polarity, from the developer prior to presenting the developer to an electrostatic latent image.

It is still a further object of the invention to eliminate the above disadvantages and suppress background by withdrawing from the developer both the low charged toner particles and the toner particles which are charged to the wrong polarity prior to presenting the developer to an electrostatic image.

A still further object of this invention is to suppress development of selected lower contrast electrostatic images by removing lower charged toner prior to being presented to an electrostatic image.

Other objects of the invention will become apparent from the following description with reference to the drawings wherein:

FIG. 1 is a view of a development system utilizing the 7 principles of this invention;

FIG. 2 is an enlarged view of the details of a toner collection mechanism utilized in the embodiment of FIG. 1;

FIG. 3 is a schematic view of the toner collection mechanism illustrated in FIG. 2;

FIG. 4 is a view of a magnetic brush development system utilizing the principles of this invention; and

FIG. 5 is a view of a modification of the embodiment of FIG. 1.

Referring to FIG. 1, a drum 10 has a photosensitive surface 12 thereon bearing an electrostatic latent image. The drum 10 is rotated in a clockwise direction by a motor 14 outlined in block form. A developer housing 16 is located adjacent the drum and contains a toner reservoir 18 having a supply of toner 20 therein and a developer sump having a supply of developer 22' therein. The developer comprises carrier and toner particles so selected that a triboelectric attraction exists between them causing the two particles to cling together and acquire an opposite charge. Each carrier has numerous toner particles attracted thereon allowing them to be transferred into contact with the photosensitive surface where the greater electrostatic attraction of the latent image will overcome the triboelectric attraction between the two developer components causing toner to be stripped off the carrier and electrostatically bonded to the charged image to effect development thereof. A bucket conveyor 24 is rotatably mounted in the housing 16 and is arranged to convey developer 22 from the developer sump upwards and deposit the same onto a chute 26. A motor 28 shown in block diagram, is positively connected to a pulley 30 to rotate the pulley and thereby the bucket conveyor. A toner pickoff rotatable roller 32 is located at the bottom of the chute 26 and has a motor 34 (shown in block diagram) positively connected thereto for rotating the same in a clockwise direction. The roller 32 collects toner particles thereon in a manner to be described hereafter. A combined scraper and guide member 36 is located in the housing and is tangentially disposed relative to the surface of the roller 32 to engage and scrape off any toner particles collected on the roller 32 and guide the same back into the developer sump without engaging the photosensitive surface.

Referring in particular to FIG. 2, the roller 32 comprises a plurality of conductive sections or segments 38 each of which alternate with a corresponding one of a plurality of conductive sections or segments 40. The sections 38 are electrically insulated from sections 40 by isulating members 42. Sections 38 are electrically connected by wires .43 to a common lead 44 which is electrically connected to a well-known slip ring assembly (illustrated in block form) which in turn is electrically connected by lead 46 to a positive terminal of a variable power source 50. The sections 40 are electrically connected by wires 45 to a common lead 47 which is electrically connected to the slip ring assembly which is electrically connected by lead 48 to a negative terminal of the power source. The power source 50 may be selectively varied by an operator for selectively varying the potential difference between the sections 38 and 40.

During rotation of the roller 32, fields will be created between the sections 38 and 40 as shown in FIG. 3. The output of the power source is selectively set by the operator to normally produce a potential difference between the sections 38 and 40 which will produce an attractive field approximately equal or slightly greater than the attractive field in the background areas of the latent image. The fields are also controlled by the relative widths of the segments 38 and 40 as well as the potential therebetween. When developer passes over the rotating roller 32, the potential between the sections 38 and 40 will be insufficient to strip toner from its corresponding carrier if the triboelectric attraction therebetween is greater than the field due to the potential between the sections 3% and 40. However, if the triboelectric attraction between the toner and its associated carrier is less than the attractive force clue to the potential between the section 3% and 40, then the toner will be stripped from its carrier and if the toner is negatively charged, it will be deposited upon the section 38.

Those toner particles which are charged to the wrong polarity carry a small positive charge thereon when a positive charged image is used and therefore are attracted by the sections lt) which is at the lower potential. As the roller 32 rotates, the scraper 36 engages the sections 38 and W to remove any toner particles deposited thereon. The toner particles fall by gravity over the scraper guide 36 back into the developer sump without engaging the photosensitive surface 12 wherein the toner particles will be mixed with the carrier particles to become triboelectrically correctly charged and redistributed by the bucket conveyor over the chute 26 and past the roller 32 and onto the photosensitive surface to develop the latent image thereon.

The connections between the motors and their associated members to be driven thereby have been omitted for clarity and it should be understood that such connections can be any type well-known in the art. The latent image is imposed on the photosensitive surface 12 and the developed latent image is transferred from the photosensitive surface to a copy paper by wellknown xerographic techniques as for instance described in U.S. Pat. No. 3,336,905.

Referring to FIG. 4, the concept of this invention may be employed with magnetic brush development systems. All of the elements which are the same as those in the embodiment of FIGS. 1-3 are illustrated with the same reference number only with a small a affixed thereto. In this embodiment, a developer mix W comprises ferromagnetic carrier particles and toner particles located in a developer sump. A magnetic roller 102 is rotatably mounted in the sump and has a portion thereof disposed within the developer mix R00 to attract developer thereto to present the same to the latent image on the photosensitive surface 12a. A motor (shown in block form) 104i is utilized to rotate the magnetic roller 102. The toner pickoff roller 32a is located adjacent the magnetic roll 102. As the magnetic roll 102 rotates developer past the pickoff roll 32a, the roll 32a will attract those particles which are charged in the wrong polarity and the weakly charged particles. The particles attracted by the roller 32a are scraped off the roller by the scraper and guide 36a and are returned back to the developer sump for re-charging and re-use.

The power source 50 can be varied by the operator to change the potential between the segments 33 and 40 in accordance with desired results. For instance, if an original contains high background, development of the background can be suppressed by increasing the potential between the segments 38 and 410 to pick off higher charged toner. Also, the original may contain unwanted pencil marks or other images thereon which are of a lesser density than the main image. The development of the unwanted image may be suppressed by varying the power source 50 to increase the potential between the segments 3% and 40 to pick off higher charged toner which would otherwise develop the unwanted image. This will also reduce somewhat the development density of the resulting image.

Referring to FIG. 5, a modification of the embodi ment of FIG. 1 is illustrated with the same elements being designated by the same reference numerals, only with a b affixed thereto. In this case, the roller 32b is located adjacent the photosensitive surface 12b and acts not only as a toner pick-off device but also as a development electrode. The roller 32b acts as a development electrode as long as there is a substantial difference in rotational speed between the surface 12b and the roller 32b whereby the development field will be the difference between the latent image charge on the photosensitive surface and the average potential on the roller 32b. In this location of the roller 32b, the weakly charged toner and the toner particles charged to the wrong polarity particles will be withdrawn from the developer mix as the mix is being presented to the latent image.

It should be realized that if it were desired to either withdraw the weakly charged toner or toner particles charged to the wrong polarity from the system, then a normally biased roller could be substituted for the roller 32 which has been designed especially for attracting not only the weakly charged particles but also the particles which are charged to the wrong polarity. Furthermore, a development electrode could be placed opposite the biased roller when the roller is located as shown in FIG. 1. Also, the potential between the sections 38 and 40 and the relative geometry thereof could be set to achieve withdrawal of either the weakly charged toner or the toner particles charged to the wrong polarity.

It should also be realized that the sections 38 and 40 may also take the form of annular parallel rings the planes of which will be transverse to or angled with respect to the axes of the roller.

Another method for collecting both the weakly charged toner particles and the toner particles charged to the wrong polarity is to substitute two oppositely rotating rollers (in the form of a nip) for the roller 32 and pass the developer therebetween. The potential between the rollers will be such that one of the rollers will collect the weakly charged toner particles thereon and the other roller will collect the toner particles thereon which are charged to the wrong polarity.

I claim:

1. In an electrostatic copier having a photosensitive surface for carrying an electrostatic latent image, a development system for developing said latent image through the use of a developer having toner particles triboelectrically attracted to carrier particles; said development system comprising the combination of means for transporting said developer along a path leading to said photosensitive surface and means along said path and upstream of said photosensitive surface for electrostatically withdrawing from said developer improperly charged toner particles which would otherwise tend to be attracted to background areas of said image; said withdrawing means including segmented member having at least one pair of conductive segments electrically insulated from each other, and means for providing an electrical potential between said segments, thereby establishing a field for selectively attracting improperly charged toner particles from said developer.

2. The structure as recited in claim 1 wherein said means for transporting said developer to said photosensitive surface comprises a chute located above the pho' tosensitive surface and means for cascading said developer onto said chute, and said means for withdrawing toner particles from said developer is located above said photosensitive surface within the path of the eascading developer.

3. The structure as recited in claim 1 wherein said means for transporting said developer to said photosensitive surface comprises a magnetic brush, and said means for withdrawing toner particles from said developer is located contiguous said magnetic brush.

4. The structure as recited in claim 1 further comprising means for returning to said developer those toner particles which are withdrawn therefrom.

5. The structure as recited in claim 1 wherein said segmented member is a rotatable roller.

6. The structure as recited in claim 5 wherein said means for providing said electrical potential between said pair of conductive segments includes means for selectively varying said potential.

7. The structure as recited in claim 1 wherein said means for providing said electrical potential between said pair of conductive segments includes means for selectively varying said potential.

8. The structure as recited in claim 1 wherein said electrical potential between said pair of conductive segments is selected to create a field for attracting from said developer toner particles charged to the same polarity as the electrostatic latent image and toner particles relatively weakly charged to the opposite polarity.

9. The structure as recited in claim 8 wherein said segmented member is a rotatable roller.

10. The structure as recited in claim 8 wherein said means for providing said electrical potential between said pair of segments includes means for selectively varying said potential.

11. The structure as recited in claim 8 further comprising means for returning to said developer those toner particles previously withdrawn therefrom.

12. The structure as recited in claim 8 wherein said means for transporting said developer comprises a chute located above the photosensitive surface and means for cascading said developer onto said chute and said photosensitive surface, and said means for withdrawing toner particles from said developer is located within the path of the cascading developer. 

1. In an electrostatic copier having a photosensitive surface for carrying an electrostatic latent image, a development system for developing said latent image through the use of a developer having toner particles triboelectrically attracted to carrier particles; said development system comprising the combination of means for transporting said developer along a path leading to said photosensitive surface and means along said path and upstream of said photosensitive surface for electrostatically withdrawing from said developer improperly charged toner particles which would otherwise tend to be attracted to background areas of said image; said withdrawing means including segmented member having at least one pair of conductive segments electrically insulated from each other, and means for providing an electrical potential between said segments, thereby establishing a field for selectively attracting improperly charged toner particles from said developer.
 2. The structure as recited in claim 1 wherein said means for transporting said developer to said photosensitive surface comprises a chute located above the photosensitive surface and means for cascading said developer onto said chute, and said means for withdrawing toner particles from said developer is located above said photosensitive surface within the path of the cascading developer.
 3. The structure as recited in claim 1 wherein said means for transporting said developer to said photosensitive surface comprises a magnetic brush, and said means for withdrawing toner particles from said developer is located contiguous said magnetic brush.
 4. The structure as recited in claim 1 further comprising means for returning to said developer those toner particles which are withdrawn therefrom.
 5. The structure as recited in claim 1 wherein said segmented member is a rotatable roller.
 6. The structure as recited in claim 5 wherein said meanS for providing said electrical potential between said pair of conductive segments includes means for selectively varying said potential.
 7. The structure as recited in claim 1 wherein said means for providing said electrical potential between said pair of conductive segments includes means for selectively varying said potential.
 8. The structure as recited in claim 1 wherein said electrical potential between said pair of conductive segments is selected to create a field for attracting from said developer toner particles charged to the same polarity as the electrostatic latent image and toner particles relatively weakly charged to the opposite polarity.
 9. The structure as recited in claim 8 wherein said segmented member is a rotatable roller.
 10. The structure as recited in claim 8 wherein said means for providing said electrical potential between said pair of segments includes means for selectively varying said potential.
 11. The structure as recited in claim 8 further comprising means for returning to said developer those toner particles previously withdrawn therefrom.
 12. The structure as recited in claim 8 wherein said means for transporting said developer comprises a chute located above the photosensitive surface and means for cascading said developer onto said chute and said photosensitive surface, and said means for withdrawing toner particles from said developer is located within the path of the cascading developer. 